1. Field of the Invention
The present invention relates to a high temperature semiconductor which is useful for an element for measuring temperature, more particularly to a highly-oriented diamond film thermistor having a fast response, and resistance to heat, radiation and chemicals, and a manufacturing process of the same.
2. Prior Art
Diamond is very hard and has a high thermal conductivity as well as an excellent resistance to heat, radiation and chemicals. Recently, it became possible to prepare a diamond film by chemical vapor deposition (CVD). Speaker diaphragms and heat sinks for semiconductor devices are being developed. Diamond free from impurities is electrically insulating, but diamond can be converted to p-type semiconductor by boron (B)-doping. The band gap of this p-type semiconductor is very large (about 5.4 eV). Moreover, its semiconducting characteristics persist at high temperatures beyond 100.degree. C. Heat resistant electric elements such as diodes and transistors using such semiconducting diamond are being developed. Another example is thermistor. Thermistor is an electronic device utilizing a property of semiconducting material that its resistance changes with temperature, and used as a temperature sensor. A thermistor most commonly used generally comprises metal oxides and is used in the temperature range up to 350.degree. C. At present, there is an interest in thermistors made of diamond, because it is stable at higher temperature (H. Nakahata, T. Imai, H. Shiomi, Y. Nishibayashi and N. Fujimori, Science and Technology of New Diamond, pp. 285-289, 1990). Diamond has a high thermal conductivity and a small specific heat. Therefore, it is expected that a thermistor utilizing diamond has a fast response to temperature changes.
In the prior art thermistor utilizing diamond, polycrystalline diamond films grown on non-diamond substrates by CVD is used. The electrical resistance of the diamond film can be easily controlled by impurity doping during the CVD process. The diamond film has advantage over the single crystal diamond because it can be produced at low cost.
In the prior art thermistor, however, a diamond film having diamond crystals grown randomly on a substrate (a polycrystalline diamond film: PCD film) is used. Such a polycrystalline diamond film contains many grain boundaries and defects. Therefore, if the PCD film thermistor is operated in air at high temperature, the PCD film is oxidized and graphitized gradually from grain boundaries, and therefore a heat resistance of the thermistor is inferior to the thermistor made of single crystal diamond. The existence of grain boundaries and defects also cause a slow temperature response. Grain boundaries and defects also act as current leakage paths and therefore the uniformity in electric properties is deteriorated. Moreover, in a PCD film thermistor, there are different crystal planes such as the (100) and (111) planes on the surface of the diamond film. Such surface structure causes different intake of impurities in different crystal planes during the growth of semiconducting diamond films by CVD or ion implantation which leads to nonuniform electrical characteristics of the thermistor.
If a single crystal diamond (SCD) is used as a substrate, a SCD film can be formed on the substrate. Problem aforementioned can be solved by using such a SCD film. SCD substrates, however, are very expensive and therefore the manufacturing cost of the thermistor becomes very high. Moreover, the surface area of a commonly available single crystal diamond substrate is only 5.times.5 mm.sup.2 and therefore a mass production of thermistor is impossible.